Power-Efficient Wireless Coverage Using Minimum Number of UAVs

Unmanned aerial vehicles (UAVs) can be deployed as backup aerial base stations due to cellular outage either during or post natural disaster. In this paper, an approach involving multi-UAV three-dimensional (3D) deployment with power-efficient planning was proposed with the objective of minimizing the number of UAVs used to provide wireless coverage to all outdoor and indoor users that minimizes the required UAV transmit power and satisfies users’ required data rate. More specifically, the proposed algorithm iteratively invoked a clustering algorithm and an efficient UAV 3D placement algorithm, which aimed for maximum wireless coverage using the minimum number of UAVs while minimizing the required UAV transmit power. Two scenarios where users are uniformly and non-uniformly distributed were considered. The proposed algorithm that employed a Particle Swarm Optimization (PSO)-based clustering algorithm resulted in a lower number of UAVs needed to serve all users compared with that when a K-means clustering algorithm was employed. Furthermore, the proposed algorithm that iteratively invoked a PSO-based clustering algorithm and PSO-based efficient UAV 3D placement algorithms reduced the execution time by a factor of ≈1/17 and ≈1/79, respectively, compared to that when the Genetic Algorithm (GA)-based and Artificial Bees Colony (ABC)-based efficient UAV 3D placement algorithms were employed. For the uniform distribution scenario, it was observed that the proposed algorithm required six UAVs to ensure 100% user coverage, whilst the benchmarker algorithm that utilized Circle Packing Theory (CPT) required five UAVs but at the expense of 67% of coverage density.


Introduction
Unmanned aerial vehicles (UAVs) have become a promising solution in supporting public safety, search and rescue operations and disaster management. In the case of natural disasters such as earthquakes, floods or tsunamis, there are chances that the communication systems' infrastructures become partially or completely disrupted. Therefore, rapid solutions are necessary to provide wireless coverage in support of rescue operations [1,2].
Generally, the main uses of UAVs as wireless aerial base stations can be classified into different categories based on their roles in different application scenarios [1, 3,4]: (i) UAVaided wireless communication due to cellular outage, where a UAV is used as a backup base station that operates at a much higher altitude to provide ubiquitous coverage when the ground base station completely goes out of service during disastrous situations [5][6][7][8].
(ii) UAV-aided wireless communication during cellular network congestion, where a UAV is used to supplement the existing ground base stations during a massively crowded special event when the cellular network is overloaded [9,10]. (iii) UAV-aided relay communication, where UAVs are used as relay nodes in providing wireless connectivity between two or more distant wireless points when there is no direct communication links or line-of-sight (LoS) due to an obstruction such as hill [11]. (iv) UAV-aided data collection, where UAVs are used to collect data from ground Internet of Things (IoT) devices [12].
In this paper, the deployment of multiple UAVs as backup base stations during natural disasters is considered. More specifically, we consider a multi-UAV-aided system to establish an emergency network that aims to maximize the wireless coverage using the minimum number of UAVs. Here, it is considered that each UAV serves both outdoor and indoor users that are either uniformly or non-uniformly distributed inside the coverage area.

Related Works
Recently, there has been an extensive amount of research related to the deployment of UAVs as aerial base stations. The optimal location of UAVs is one of the main issues that must be addressed in the case of UAV-aided wireless communication due to cellular outage or during cellular network congestion. The problem of finding the optimal location of a UAV is formulated with various objective functions such as to maximize wireless coverage or network throughput, to maximize user coverage probability and to minimize power consumption.
The authors in [7,8,10,[13][14][15][16] studied the case of the deployment of a single UAV as an aerial base station to serve outdoor users with different objective functions. However, these studies utilized the path loss model in [17], where a statistical propagation model was proposed to predict the Air-To-Ground (ATG) pathloss between UAVs in low-altitude platforms (LAPs) and ground nodes. The authors in [8] studied the problem of finding the optimal altitude of a single UAV that aimed to provide maximum wireless coverage with minimum UAV transmit power. The authors also studied the case of two UAVs that provided maximum coverage in the presence and absence of interference. The study in [13] proposed a model to find an efficient UAV 3D position that aimed to maximize the network throughput. In [14], the authors proposed an optimal 3D placement algorithm for the deployment of a single UAV base station that aimed to maximize the total number of covered users by imposing the minimum required UAV transmit power constraint. The authors in [16] studied an on-demand UAV placement problem for arbitrarily distributed users. The problem was formulated with the objective of maximizing the covered users for different user densities with guaranteed data rates.
On the other hand, the authors in [7] proposed a model to find an efficient 3D placement of a single UAV that served indoor users by utilizing the outdoor to indoor pathloss model of [18] certified by the International Telecommunication Union (ITU). The objective was to minimize the total required UAV transmit power such that all indoor users were covered. Furthermore, the authors in [10] studied the problem of finding the optimal altitude of a single UAV that served outdoor and indoor users simultaneously and aimed to minimize the total required UAV transmit power and maximize the coverage area.
The work presented so far is related to the deployment of a single UAV that serves outdoor and/or indoor users within small coverage areas. Thus, in the case of users within a large coverage area, the deployment of multiple UAVs is required.
The studies in [19][20][21] considered the cellular networking scenario where UAVs were deployed to support the connectivity of existing terrestrial wireless networks. The study in [19] proposed an approach to deploy multi-UAVs coexisting with a ground base station to provide wireless coverage for users in a crowded region. In this work, the efficient number and the 3D placement of UAVs were found in such that the traffic demands were satisfied, with arbitrary user distribution, and the system sum-rate was maximized. In addition, this work considered the problem of co-channel interference. In [20], the authors proposed a coordination and cooperation model where a UAV was used to assist the terrestrial cellular network. Specifically, the authors developed a cooperative UAV clustering approach to offload ground mobile terminals from ground cellular base stations to cooperative UAV clusters. Meanwhile, the work in [21] addressed the issue in the backhaul links between drones and ground base stations. More specifically, the authors in [21] proposed a 3D UAV placement and trajectory model. Bezier curves were utilized to achieve the best coverage for clusters of ground terminals.
Another main issue to be considered in the deployment of multi-UAVs is finding the number of UAVs to be deployed. The studies in [9,[22][23][24] considered the deployment of multi-UAVs equipped with a directional antenna that has a circular coverage pattern. The authors in [25] proposed an approach using a heuristic algorithm to find the minimum number of UAVs along with their placements to provide coverage for outdoor users. The coverage area was divided into equal regions and the users were distributed uniformly with different densities. In [23], the authors proposed an optimal 3D deployment strategy of multi-UAVs that used Circle Packing Theory (CPT). In this work, the optimal 3D location of the UAVs were determined with the aim to maximize the circular coverage area. A similar approach was presented in [9], where the authors proposed to utilize CPT to find the number of UAVs with the aim to maximize the coverage density while ensuring the coverage area of each UAV did not overlap. However, the authors considered three different shapes of wireless coverage area, namely square, rectangular and circular regions. Here, the CPT was utilized in tandem with the 3D efficient placement algorithm.
Another interesting approach is the deployment of multi-UAVs by employing a clustering algorithm [26][27][28][29][30]. The authors in [26] proposed an algorithm to position UAVs in order to complement the macrocell infrastructure. The K-means clustering algorithm was used to partition user equipments (UEs) into K subsets, and a decision was made on which subsets were to be serviced by UAVs. The centroid of the subset was set as the 2D location of the UAV which served the UEs within the subset to offload from the macrocells. In [27], the authors studied a similar approach as the work in [26]; however, the positioning of multi-UAVs and the association of the UEs were jointly optimized and aimed to maximize the number of UEs and satisfy the UE's experienced data rate (represented via bandwidth allocation). Two jointly optimized algorithms based on Particle Swarm Optimization (PSO) and the Genetic Algorithm (GA) were proposed. Both proposed algorithms improved the UE's satisfaction with the provided data rates when compared with the proposed algorithm that invoked a K-means algorithm.
However, both works in [26,27] considered the 2D placement of UAVs that served the associated UEs.
The authors in [28] studied the 3D placement of a single UAV as an aerial base station to serve indoor users alone inside a high-rise building. The problem was formulated with the aim to minimize the number of UAVs required to serve all indoor users. This work considered the indoor users that were distributed uniformly with the pathloss model of [18]. The proposed solution used a K-means clustering algorithm to partition the indoor users into an initial k cluster and then exploited PSO to find an efficient 3D position of UAV subjected to the constraint that the total transmit power was smaller than the threshold value. A single UAV was assigned to each cluster. The number of clusters was increased if the total transmit power constraint was not met. This process was performed iteratively until the total transmit power constraint was met.
In [29], the authors proposed a UAV-aided emergency rescue network where each UAV served as a wireless base station. The problem was formulated with the objective to minimize the number of UAVs to cover all points of interest (POIs), which were locations with a large number of people, for example, schools, hospitals or parks. This work considered the location of POIs that were distributed uniformly and were located outdoors and indoors, with the pathloss model of [17,18], respectively. The authors proposed a similar solution as the work in [28], where an iterative process was performed to partition the POIs into k clusters using a K-means clustering algorithm and finding an efficient 3D position of the UAV. However, this work exploited a Genetic Algorithm (GA) to find the efficient 3D position of a UAV in providing wireless coverage to POIs that were located outdoors and indoors.
In [30], the authors proposed a multi-UAV deployment strategy for resource allocation in a UAV-enabled mobile edge computing (MEC) network. This work aimed to minimize the sum power consumption of UEs and UAVs that included both communication-related power and mechanical power with latency and coverage constraints. The sum power minimization problem was decomposed into three subproblems on user association, computation capacity allocation and location planning. The proposed algorithm iteratively invoked the algorithm to solve the three subproblems. In this work, the Fuzzy C-means clustering algorithm was proposed to solve the joint user association and location planning subproblems. The proposed algorithm could efficiently reduce the sum power consumption after three iterations, indicating the reduction in the number of UAVs used.
Meanwhile, in [24], the authors studied the efficient placement of multi-UAVs in such that the user coverage probability was maximized and inter-cell interference (ICI) was avoided. More specifically, the authors proposed a non-overlapped circle placement method to find the optimal UAV placement in such that the circle covered the maximum number of users by considering the required total transmit power. In this work, the outdoor users were randomly distributed using spatial points processes (SPPs) and were partitioned into K clusters using a K-means algorithm; subsequently, the optimal UAV placement was determined using the proposed algorithm in such that the user coverage probability was maximized while the total transmit power was minimized. The user coverage probability and the power efficiency were further improved using the proposed iterative algorithms.
However, the clustering algorithm used in [24,26,28,29] exploited a K-means algorithm to partition users into K clusters.
A K-means algorithm is relatively a simple algorithm and is easy to implement, which makes it the most popular algorithm that is used in several fields. However, a K-means algorithm is very sensitive to the initial cluster centers. More specifically, in K-means algorithms, the initial cluster centers are chosen randomly; this may cause three possible problems: (i) The final clustering results are not unique. For different runs of the same input data, the algorithm produces different clusters [31]. (ii) The initial cluster center has an influence on the number of iterations. The number of iterations required for the algorithm to converge is high if the chosen initial cluster center is located far away from the final cluster center [31]. (iii) The algorithm may converge in local minimum instead of global minimum, which results in a sub-optimal solution [32,33]. Furthermore, the K-means algorithm is sensitive to outliers and noisy data. The outliers have an influence on the clustering of data points that results in some data points being clustered incorrectly [32,33]. Moreover, the algorithm produces clusters that are not symmetric, where the clusters have unequal sizes and densities (unequal numbers of data points in each cluster) [34].
Various methods have been proposed in the literature to mitigate the drawbacks of K-means algorithms. The authors in [35,36] proposed a GA-based clustering algorithm as a solution to the problem that a K-means algorithm may get stuck at sub-optimal solutions due to a poor randomly chosen initial cluster center. Meanwhile, the application of PSO in data clustering was discussed in [32]. It has been demonstrated that both GA and PSO algorithms overcome the problem associated with the K-means algorithm that tends to trap in local optima [32,35,36].
Meanwhile, the work in [28,29] considered the location of users/POIs that were distributed uniformly. In [26], three set of UEs were distributed with three different distributions, namely Poisson, random and uniform distribution, whilst in [24], the authors considered the users to be distributed using SPP.
With regard to the energy efficiency approach, the authors in [37] proposed an iterative algorithm to solve the energy efficiency problem with the objective to maximize the ratio of the ergodic total data size to the total energy consumption that included both transmit power and propulsion power to hover. The problem was decomposed into two subproblems on finding optimal coordinated power allocation and finding optimal hovering time scheduling. The proposed iterative algorithm resulted in higher energy efficiency as the number of UAVs increased, due to higher diversity gain and flexibility in the coordination of aerial small cells. This observation was obtained because more UAVs resulted in less hovering time for a fixed total transmit energy. Thus, the transmit power for each UAV was higher; hence, the total data size increased. In [38], it was stated that the energy consumption of a UAV refers to the conventional communications-related energy consumption and propulsion energy consumption. However, this work focuses on the communications-related energy consumption alone.
In this paper, we propose a power-efficient algorithm that maximizes the coverage area using the minimum number of UAVs that aims to minimize the required UAV transmit power while satisfying the required users' data rate. The proposed algorithm considers the deployment of multi-UAVs that serve outdoor and indoor users by iteratively invoking a clustering algorithm and an efficient 3D UAV placement algorithm.
More specifically, we study the proposed algorithm that invokes three variants of clustering algorithms that are developed based on a K-means algorithm, PSO and GA for partitioning both outdoor and indoor users into k clusters. Here, we define user partitioning as dividing coverage area into k small subareas. In this study, we consider two scenarios where users are distributed uniformly and non-uniformly using beta function. Subsequently, the proposed algorithm invokes three variants of efficient 3D UAV placement algorithms that are developed based on PSO, GA and Artificial Bees Colony (ABC).

Paper Contribution
The contributions of this paper are summarized as follows: • K-means and meta-heuristic clustering algorithms, based on PSO and GA, respectively, are utilized for partitioning outdoor and indoor users into clusters which correspond with partitioning the disaster-affected area with the condition that the UAV transmit power for each cluster is minimized. The employment of the meta-heuristic algorithm is superior in comparison with the K-means based clustering algorithm in terms of the cluster quality, where the resulting clusters are symmetrical. • The efficient UAV 3D placement algorithm based on the ABC algorithm is proposed, with the aim to minimize the required UAV transmit power while satisfying the data rate requirement. The employment of each of the three variants of the efficient UAV 3D placement algorithm, namely PSO-based, GA-based and ABC-based algorithms, are evaluated in terms of the computational complexity which is manifested in terms of its execution time taken. • A power-efficient algorithm is proposed that iteratively invokes a clustering algorithm and an efficient UAV 3D placement algorithm that aims to minimize the number of UAVs to serve outdoor and indoor users simultaneously, while minimizing each UAV transmit power. The proposed algorithm attained 100% coverage density, which corresponds with providing wireless coverage to all users that are uniformly and non-uniformly distributed using the minimum number of UAVs. Furthermore, the proposed algorithm that invokes a PSO-based clustering algorithm resulted in a lower number of required UAVs that served all outdoor and indoor users compared to that when the K-means clustering algorithm was employed.
Section 3 introduces the system model. Section 4 presents the problem formulation and the proposed algorithm to find the minimum number of UAVs that aims to maximize wireless coverage by imposing the constraint to minimize the required UAV transmit power. This is followed by the clustering algorithms that are invoked to partition users by the proposed algorithm in Section 5. Then, Section 6 presents three variants of efficient UAV 3D placement algorithms based on PSO, GA and ABC algorithms. Section 7 quantifies the comparison performance of the three clustering algorithms variants, as well as the performance of the proposed power-efficient algorithm that iteratively invokes a clustering algorithm and an efficient UAV 3D placement algorithm for the case when users are distributed uniformly and non-uniformly. Finally, Section 8 concludes the paper.

System Model
Consider an area where a natural disaster occurs, denoted as S. Figure 1 illustrates the affected area, S, with minimum and maximum points of (x min , y min ) and (x max , y max ), respectively. This work aims for an power-efficient deployment strategy of multi-UAVs that simultaneously serves all outdoor and indoor users within the disaster-affected area, S. More specifically, in this work, three different clustering algorithms are invoked for use in partitioning the users which corresponds to partitioning the coverage area, S, into n subareas. One UAV is assigned to serve each subarea, S n . Moreover, this work considers two scenarios of user distribution inside the coverage area, namely uniform distribution and non-uniform distribution using beta random distribution, denoted as function f (x, y). In this work, the MATLAB beta random generation function betarnd (A, B, m, n) is used to generate an m-by-n array. This array contains random numbers from the beta distribution with parameters A and B, with A = 1, B = 1, m = 1 and n = number of users inside coverage area. Figure 1 illustrates the distribution of outdoor and indoor users within the disaster-affected area, S, which is represented as blue circles and red crosses, respectively.

Path Loss Models
This section presents the Air-To-Ground (ATG) [17] and Outdoor-to-Indoor [18] path loss models that are utilized when considering the deployment of UAVs to serve outdoor and indoor users, respectively.
For the ATG channel modeling in an urban environment, both the line-of-sight (LOS) links and the non-line-of-sight (NLOS) links between the UAV and the ground users are considered. The propagation conditions in the probabilistic LAP model [17] have been extensively used for coverage analysis and for finding the optimal UAV position [8,23]. The probability of LOS and NLOS can be calculated based on the relative locations between UAVs and ground outdoor users. The average path loss, PL out (dB), of the ATG channel model can be formulated as: where the path loss for the LOS link, L LOS , and the NLOS link, L NLOS , as well as the probability of LOS, P LOS , and NLOS link, P NLOS , are given as [17]: , where c is light speed, f c is the carrier frequency and η LOS , η NLOS are additional losses depending on the environment. Meanwhile, a and b are constants that depend on the environment. θ is the elevation angle between the UAV and ground user, h is the altitude of the UAV and d is the distance between the UAV and the ground user. Meanwhile, the Outdoor-to-Indoor path loss model of [18] is utilized when considering wireless coverage for indoor users, which is given as: where PL PFS is the path loss in the free space, PL PB is the penetration loss of the building and PL I N is the indoor path loss, d 3D is the Euclidean distance between the UAV and the indoor user i, f c is the carrier frequency, θ is the elevation angle, d 2D in is the 2D indoor distance between the UAV and indoor user i and a 1 , a 2 , a 3 and a 4 are environmental constant values.

Problem Formulation
In this section, the deployment of multi-UAVs to provide wireless coverage for outdoor and indoor users using the minimum number of UAVs during a natural disaster is formulated as an optimization problem.
Consider a disaster-affected area, S. Let the dimension of the area be denoted by [0, x max ] × [0, y max ]. A set of M out outdoor users and M in of indoor users are distributed inside the area. The 2D user location is represented with their location vector P = (p x , p y ). The problem of finding the minimum number of UAVs that provide wireless coverage for all outdoor and indoor users inside S and their efficient 3D placements can be formulated as follows: where K is defined as the total number of UAVs that are deployed to serve all outdoor and indoor users inside S and (x u , y u , z u ) is the UAV 3D placement that minimizes its transmit power. The first constraint of Equation (4b) guarantees that the total power required to cover all users is less than the threshold power of UAV P U AV max , where the total required power for outdoor and indoor users that satisfies the minimum data rate R b can be represented as follows: The constraint of Equations (4c) and (4d) ensure that there is no overlapping between all users that are served by k th UAV and other users that are not served by the k th UAV. In Equation (4c), M k th out and M k th in denote the the outdoor and indoor users that are served by k th UAV, whilst (N out + N in ) defines the total number of users inside S. In Equation (4d), P j denotes the 2D location vector of each user that is served by the k th UAV, whilst P¯j denotes the 2D location vector of each user that is not served by the k th UAV.
In the constraint of Equation (4e), the total available bandwidth, B, is divided evenly between UAVs, where B k denotes the bandwidth that is allocated for each UAV. The constraint of Equations (4f)-(4h) ensure that all UAVs are located within the range of minimum and maximum values inside the coverage area.
In this paper, we assume that the interference is implicitly modeled as noise. In the system model, Frequency Division Multiple Access (FDMA) is used. It is assumed that each UAV allocates equal channel bandwidth to ground users, and in order to avoid interference, each channel is assigned to one user.
Clearly, finding the minimum number of UAVs that serve all users inside S and their efficient 3D placements such that the total transmit power of each UAV is minimized makes the problem very complicated. Therefore, this optimization problem is an NP-hard problem [39] and can be solved using meta-heuristic algorithms. Algorithm 1 shows the pseudocode of the proposed algorithm to solve the formulated problem similar to the approach in [28,29]. More specifically, the proposed approach to solve the formulated problem can be performed in the following stages, as illustrated in Figure 2: Initially, the number of UAVs that are used to serve all users inside the coverage area S is set as k = 2.

2.
Then, the proposed clustering algorithm is invoked to partition the users into k clusters. The proposed three variants of clustering algorithm based on K-means algorithm, PSO and GA are presented in Section 5.

3.
The UAV 3D placement for each k cluster is determined by invoking the proposed efficient UAV 3D placement algorithm. Section 6 presents the three variants of the UAV efficient 3D placement algorithm. 4.
The required total transmit power to provide wireless coverage to all users inside S is determined using Equation (4b). If the UAV transmit power ≥ P U AV max , then the value of k is increased by 1. In other words, the number of clusters of users inside S is increased by 1.

5.
An iterative process of stage (2) to (4) is performed until the constraint of Equation (4b) is met. In this work, we use P U AV max = 1 watt. The computational complexity of the proposed heuristic approach is the summation of the complexity of the clustering algorithm and the complexity of the efficient UAV 3D placement algorithm.
1. STEP 1: Input: Coverage region S; p = (p x , p y ) set of users location; k clusters. 2. STEP 2: Repeat: 3. Partitioning users inside S into k clusters using K-means, PSO and GA algorithms. For each cluster an efficient UAV placement is found using PSO and GA Algorithms such that: 4.
Increase the number of clusters by 1. 9.
Go to Step 2. For the sake of benchmarking, the multi-UAV deployment approach that utilizes CPT is used for comparison with the proposed power-efficient algorithm. The benchmarker multi-UAV deployment is only considered for the scenario where all users are uniformly distributed.
More specifically, the performance of the proposed power-efficient algorithm is compared with the efficient 3D placement of multi-UAVs that utilizes CPT to serve users in a square shape coverage region of [9]. The benchmarker multi-UAVs deployment approach invokes CPT to find the optimal packing of n c non-overlapped and identical circles into a unit square. Then, the altitude of each UAV is found using the algorithms presented in Section 6.
The problem of packing n c identical circles inside a unit square with the objective to maximize the radius r d of the packed circles such that the coverage density is maximized can be formulated as [9]: where (x i , y i ) is the center coordinates of the ith circle, (x i − x j ) 2 + (y i − y j ) 2 is the Euclidean distance between the centers of circles i and j and r d is the radius of each packed circle. All packed circles are guaranteed to lie inside the square and there is no overlapping between packed circles by imposing the constraint of Equations (6b)-(6d). The density d n of packing n c identical and non-overlapped circles with radii r d inside a unit square is defined as the ratio of the packed n c circles area to the square area as in the following Equation [9]:

Clustering Approaches
This section presents two different clustering approaches which may be referred to as iterative distance-based clustering and meta-heuristic clustering algorithms. The objective of the clustering problem formulation is to minimize the Euclidean distance between each user and the centroid, as presented in Section 5.1. Sections 5.2 and 5.3 present the iterative distance-based clustering and meta-heuristic clustering algorithms, respectively.
One of the most well-known clustering algorithms that falls in the iterative distancebased clustering algorithm category is K-means. In [28,29], a K-means-based clustering algorithm was employed to partition users and POIs in the case of the deployment of multi-UAVs as aerial base sations. As discussed in Section 2, the clustering problem of the K-means algorithm can also be mitigated by utilizing meta-heuristic algorithms.
Thus, this paper extends the clustering approach based on the meta-heuristic clustering algorithm by invoking PSO, as presented in Section 5.3.2. The performances of the three clustering algorithms are compared in terms of algorithm robustness to the outliers and computational complexity, as presented in Section 7.

Mathematical Formulation of the Clustering Problem
The clustering algorithm involves the process of gathering similar data points into the same cluster. Therefore, a similarity metric between two points must be defined. Euclidean distance is the most common distant metric used in clustering algorithms that aims to minimize the total variations within each cluster [40,41]. Thus, the clustering problem can be formulated with the objective to minimize the sum of squared Euclidean distances between each data point with the cluster center, as follows: where x n is the nth value of the data set ∈ N and the center of the cluster k, and c k is the average value of all points in cluster k which can be determined using Equation (9).
where W nk is a decision weight ∈ 0, 1 , W nk = 1 if the point n belongs to cluster k and otherwise if the n point does not belong to the cluster k.

Iterative Distance-Based Clustering (K-Means)
A K-means algorithm is a partitioning clustering algorithm used to group data or objects into clusters [31] which was developed by J. B. Mac Queen in 1967 [42]. A K-means algorithm starts by randomly selecting k initial means as the cluster centers, referred to as centroids. Then, this algorithm calculates the Euclidean distance from each data point to these centroids, and a cluster is formed based on the shortest Euclidean distance from the data point to the cluster centroid. Once the data points are grouped into clusters, the centroid is replaced by a new mean value that is calculated based on the mean of the points that belong to the cluster. These tasks are performed for several iterations until the algorithm converges and produces K final means [31,43]. However, in this work, the number of clusters are set using the proposed heuristic algorithm of Algorithm 1, as described in Section 4. The K-means algorithm is described by the pseudocode of Algorithm 2.

Algorithm 2:
The K-means clustering algorithm.
Result: A set of K clusters Input; k: Number of desired clusters Data set D = d i |i = 1, . . . , n ; n set of data points. c k : set of centers k = 1, . . . , K. u k : cluster position that minimizes the distance from the data points to the cluster k = 1, . . . , K Initialization; c i = random(num): Arbitrarily choose k items from D as initial centroids; Repeat for ∀j = 1:n Assign i to Cluster c k according to the minimum distance from c k center The computational complexity of a K-means algorithm refers to the total number of Euclidean distance computations. More specifically, in each iteration, for a data set D = d i |i = 1, . . . , n , where n is the total number of data points and k is the number of clusters, the computation complexity is O(nk). Thus, for N it number of iterations, the computational complexity is O(nkN it ).

Meta-Heuristic Clustering Algorithms
Several meta-heuristic-based clustering algorithms have emerged with nature-inspired designs, namely GA and PSO [33,44]. As discussed earlier, several works on the deployment of multi-UAVs as an aerial base station have employed K-means clustering algorithms to partition users. Thus, in this work, we study the deployment of multi-UAVs by invoking the meta-heuristic-based clustering algorithm that provides wireless coverage for both outdoor and indoor users.

Genetic Algorithm (GA)
A Genetic Algorithm (GA) is an optimization technique based on the principles of evolution and natural genetics, which can be used to solve NP-complete global optimization problems. It was demonstrated that the GA-based clustering algorithm provided superior performance in solving clustering problems [35,36].
In the initialization stage, an initial population of k number of individuals is created, which are referred to as chromosomes. These individuals represent the legitimate solutions of the given optimization problem. In the case of the clustering problem presented in Section 5.1, these individuals represent the cluster centers which initially are chosen randomly. Each legitimate solution will be evaluated by its fitness, which is linked to the objective function of the clustering problem. Then, the termination criterion will be examined. In this work, the number of iterations, N it , refers to the number of generations, which is used to be the termination criterion. If the termination criterion is not met, the highest-fitness individuals are selected to generate the next generation. In this work, the fitness proportional selection is used. Furthermore, the selected individuals, which are referred to as parents, will undergo genetically inspired operators, namely crossover and mutation. Thus, new individuals are generated with improved performance.
The fitness of the new set of legitimate solutions or individuals will be evaluated and the termination criterion will be re-examined. The three GA operators, namely selection of the fittest, crossover and mutation will be repeated during each iterative procedure until the termination criterion is met. After this stage, the algorithm produces a set of the best individuals with the highest fitness, which are the solution to the clustering problem, namely the cluster centers [35]. Algorithm 3 presents the pseudo code for the GA-based clustering algorithm. center c j such that: ||x i − c j || < ||x i − c p ||, ∀p = 1, 2, . . . , k 2. For each cluster the new center is c * The new C * i is replaced by the old center c i Select parents using proportional selection for the next Generation (G) Apply the Crossover (C) operator for p(t) Apply the Mutation (M) operator for p(t) The computational complexity of the GA-based clustering algorithm relies on the total number of Euclidean distance computation, as well as the three GA operators, namely selection of the fittest, crossover and mutation. In the selection stage, the best individual will be selected using the fitness proportional selection to be the parents for the new generation of k individuals using the crossover and mutation operators. The fitness proportional selection or roulette wheel selection has the computational complexity of O(log(k)). Prior to the selection stage, the Euclidean distance of each data point from each of k cluster center, c k , will be computed. Hence, it takes O(nk) computations. Thus, for a N it number of iterations, the computational complexity of the GA-based clustering algorithm is O(N it · nk log(k)).

Particle Swarm Optimization (PSO)
In this work, the new clustering algorithm that invokes Particle Swarm Optimization (PSO) is introduced. The PSO is a population-based evolutionary algorithm that was developed by Kennedy and Eberhart in 1995 [45]. This robust algorithm simulates animals' social behavior and movement of animal swarms such as schools of fish or flocks of birds, and it is able to solve complex optimization problems.
In PSO, each swarm member is referred to as a particle, where each particle is the candidate solution to the optimization problem. During the movement of the particle swarm, the members of the swarm interact and influence each others' state. More specifically, each member of the swarm moves cooperatively, which forms the speed and direction of the whole swarm in finding the optimal solution. In the PSO algorithm, the speed and direction are referred to as the velocity and position of each particle which are updated according to its own experience and also that of a neighboring particle. Thus, this algorithm combines both the local and global search in finding the optimal solution.
At the beginning of the PSO-based clustering algorithm for the problem formulated in Section 5.1, the positions of k particles are initialized randomly and the velocities of the k particles are set to 0. Here, each particle is referred to as the cluster center. Then, the fitness of each particle in the swarm is evaluated based on the objective function of the clustering problem.
For every iteration, each particle compares its fitness with its previous best fitness; the highest fitness is set as the personal best, also known as local best, Lbest value. Then, the best fitness, Lbest, of each particle is also compared with the Lbest of other particles in the swarm, and the swarm global best is updated with the greatest fitness, which is known as global best, GBest [46].
Subsequently, the velocity of each particle is modified towards its Lbest and GBest using Equation (10), and its new position is calculated using Equation (11). The ith particle in the swarm changes its velocity and position according to the following equations: Position i (t + 1) = Position i (t) + Velocity i (t + 1) In other words, each particle moves towards its previous best, Lbest, position and the global best, GBest, position in the swarm. This process is repeated until the termination criteria is met, that is after the given maximum number of iterations. Algorithm 4 illustrates the pseudocode for the PSO-based clustering algorithm.

• PSO Complexity
Similar to the definition of computational complexity for the K-means and GA algorithms presented in the previous Sections 5.2 and 5.3.1, respectively, the computational complexity of the PSO-based clustering algorithm refers to the total number of Euclidean distance computations and the additional computations in the algorithm. More specifically, in each iteration, the Euclidean distance of each data point from each k cluster center, c k , will be computed and additional computations update particles' velocities and positions. Hence, the computational complexity can be denoted as O(nkp). Thus, for N it number of iterations, the computational complexity is O(nkpN it ).

Algorithm 4: Clustering using Particle Swarm Optimization.
Result: A set of K Clusters Input; k: Number of desired clusters Data set D = d i |i = 1, . . . , n ; n set of data points. c k : set of centers k = 1, . . . , K. Initialization; For each particle randomly initialized position and velocity; Initialized each particle with c k randomly; for iterations:1:max_iterations do for Particle:1:All Particles do for {d i |i = 1, . . . , n} do Compute Euclidean distance of d i with all cluster center: Assign d i to the cluster with nearest c k ; end Update Lbest and GBest for each particle Update velocity for each particle using Equation (10) update the position of the particle using Equation (11) end end

Efficient UAV 3D Placement Algorithms
As discussed in Section 4, in each iteration of the proposed heuristic algorithm, the efficient UAV 3D placement algorithm is invoked after the clustering process using one of the algorithms of Section 5. More specifically, this algorithm is used to determine efficient UAV 3D placement in each cluster, k.
The problem of finding an efficient UAV 3D placement is formulated with the objective to minimize its required transmit power that satisfies the users' minimum data rate. The formulation of this problem can be found in [9,10].
Due to the intractability of the problem, efficient UAV 3D placement algorithms that invoked PSO and GA were proposed in [9,15], respectively. This paper extends the solution by invoking ABC algorithm.
Readers are referred to the discussions presented in [9,15] for detailed discussion in finding the efficient UAV 3D placement by invoking PSO and GA algorithms, respectively.

Problem Formulation
In this work, it is considered that UAV transmits data to M out outdoor and M in indoor users inside cluster k at a desired data rate, r, where, (M out + M in ) is the total number of users inside a cluster, k (coverage subarea) and each user has a channel bandwidth equal to A/(M out + M in ), where A is the UAV transmission bandwidth, L i is the path loss between UAV and users i and N p is the noise power. The total required transmit power of the UAV to satisfy the data rate, r, for all users inside cluster, k can be formulated as: Thus, the problem of finding an efficient UAV 3D placement in providing wireless coverage for all users within each cluster such that the total required UAV transmit is minimized can be formulated as follows [9,10]: Here, the first three constraint equations represent the minimum and maximum allowed 3D placement for x U AV , y U AV and z U AV . In the fourth constraint equation, P max is the maximum allowable power, where P t,max is the maximum transmit power of UAV.

Artificial Bees Colony (ABC)
The Artificial Bees Colony (ABC) algorithm is a meta-heuristic algorithm based on the foraging behavior observed in honey bee swarms, which was introduced by Dervis Karaboga in 2005 [47].
In the ABC algorithm, the colony consists of three groups of bees, namely employed bees, onlooker bees and scout bees. Algorithm 5 shows the pesudocode of the efficient UAV 3D placement that invokes ABC algorithm. The algorithm consists of the following main steps: 1. Initialization: In the initialization phase, N pop random solutions are generated. In the ABC algorithm, a solution to the optimization problem is referred to as food source, θ i . 2. Employed bee phase: In this phase, each employed bee which has been assigned to a food source, θ i searches the neighboring region to seek the best food source. The best food source is selected using greedy selection. More specifically, in this phase, the employed bee seeks a new food source, that is denoted as υ i , around the assigned source. Then, the employed bee evaluates and compares the quality of the nectar of the assigned food source, θ i , and the new food source, υ i . If the new food source, υ i , results in better nectar quality, then the food source, θ i , will be replaced by υ i ; otherwise, θ i remains in the population. This selection process is known as greedy selection. The nectar quality evaluation refers to the evaluation of the objective function to the problem of finding UAV 3D placement. 3. Onlooker bee phase: Then, each employed bee returns to its hive and shares the food source location with the onlooker bees that are waiting in the hive. In this phase, the quality of the nectar from all of the employed bees is evaluated. The onlooker bee selects the food source by applying the roulette wheel selection. Then, the onlooker bee searches the neighboring region of the selected food source further. The onlooker bee performs a similar selection process in the employed bee phase where the best food source is selected using greedy selection, where the better one survives in the population. 4. Scout bee phase: If a food source cannot be improved any more, the food source is abandoned or eliminated from the population. In this work, the abandonment limit parameter is defined as 0.6 × nVar × N pop , where nVar is the dimension of the solution and refers to the 3D coordinate of (x, y, z) and N pop is the population size. This is carried out by replacing it with a random number. The employed bee whose food source has been abandoned becomes a scout bee and is assigned to a random new food source. 5. Termination criteria: If the termination criterion is not met, the employed bee phase, the onlooker bee phase and the scout bee phase will be repeated. In this work, the maximum number of iteration, N it is set as the termination criteria. The best food source will remain in the population as the best solution to the optimization problem.
Moreover, Figure 3 presents the flowchart of the ABC algorithm.  Assign the employed bees to their respective food sources Calculate the fitness of the new food source Apply Greedy selection process Assign the onlooker bees to the selected food sources with the best quality of nectar using roulette wheel selection Identify the food source to be abandoned Assign the scout bee to randomly select new area to search for new food source Memorize the best food source that results in the best nectar quality (Best food source found so far) Until: The termination criteria is met. Output: The Best Solution achieved.

Simulation Results
This section is structured as follows. Section 7.1 presents the performance comparison of clustering algorithms in Section 5 to partition all users inside the disaster-affected area, S. Then, the performance of the proposed power-efficient algorithm is presented in Section 7.2.

Performance Comparison of Clustering Algorithms
In this section, we comparatively study the performance of the iterative distancebased and meta-heuristic based clustering algorithms that are presented in Section 5. The simulation results were obtained when each clustering algorithm was employed to partition outdoor and indoor users that were uniformly distributed. The user partitioning corresponds to the partitioning of the disaster-affected area, S. Figure 4 shows the clustering results using the three clustering algorithms where the users are partitioned into six clusters. The users in each cluster are indicated with the same color. It can be seen from Figure 4a that the K-means algorithm produces clusters that are not symmetric, as shown by the clusters marked with the blue square. This observation is consistent with the results presented in [34]. Meanwhile, the meta-heuristic based clustering algorithms, namely the PSO-based and GA-based algorithms, form symmetric clusters.
It can also be observed in Figure 4a that an outlier that is marked with a red square affects the clustering process of the neighboring data points. The outlier resulted in a heavily unbalanced cluster, as marked with blue square. In other words, this indicates that the outlier resulted in some data points being clustered incorrectly [32]. Meanwhile, the same outlier data point (marked with red square) did not affect the clustering results when PSO-based and GA-based algorithms were employed, as shown in Figure 4b,c.
Moreover, it can also be seen from Figure 4b,c that meta-heuristic algorithms lead to the same clustering results [31].
The efficiency of each clustering algorithm is evaluated in terms of its execution time. In this work, the simulations were conducted using a laptop with the following specifications: Intel core i7-4710HQ CPU with 3.1 GHz processor and 8 GB RAM in Windows 10 OS. Table 1 presents the execution time taken to partition all users that are uniformly distributed into K clusters using K-means, PSO-based and GA-based clustering algorithms. The execution time taken by each clustering algorithm reflects the computation complexity defined in Section 5.1. More specifically, the K-means algorithm took the shortest time to complete the clustering process compared to the meta-heuristic-based clustering algorithms. Meanwhile, the employment of the GA-based clustering algorithm resulted in inferior performance compared to that of the PSO-based clustering algorithm. More specifically, the GA-based algorithm took a longer time to partition all users into clusters.  Therefore, the performance of the proposed power-efficient algorithm of Algorithm 1 in Section 4 is evaluated by invoking K-means clustering and PSO-based clustering algorithms. Moreover, the GA-based clustering algorithm produces similar clustering results as that of the PSO-based clustering algorithm, but at the expense of higher complexity due to the crossover and mutation operators.

Performance of Power-Efficient Algorithm
This section presents the performance of the proposed power-efficient algorithm of Algorithm 1 in Section 4. Table 2 summarizes the parameters used in the simulation.
In this study, we consider the dimension of the coverage area, S, to be 1000 m × 1000 m. The proposed power-efficient algorithm is evaluated in two scenarios where all users are distributed with two distributions, namely uniform and non-uniform distributions, as shown in Figures 5 and 6, respectively. In this simulation, the total number of users is 100, which is composed of 50 outdoor users and 50 indoor users.
In each scenario, the power-efficient algorithm invoked the K-means and PSO-based clustering algorithms of Algorithms 2 and 4, respectively, to partition the users. Then, the three variants of efficient UAV 3D placement algorithms of Section 6 that invoked PSO, GA and ABC algorithms were employed. The clustering and the efficient UAV 3D placement algorithms were iteratively invoked to find the minimum number of UAVs that provide wireless coverage to the disaster-affected area, S, in such that the UAV transmit power is minimized and the users' data rate is satisfied. In this study, the minimum users' data rate is 1 Mbps and the minimum UAV altitude z min = 60 m. The minimum UAV altitude value during SAR operation is 60 m. This value is less than 120 m, the allowable height under the Drone Safety Rules [48].
More specifically, the clustering and the efficient UAV 3D placement algorithms were performed iteratively until the the UAV total transmit power constraint of Equation (4b) was met, as described in Section 4. In other words, the iteration was terminated if the UAV transmit power required to cover all outdoor and indoor users inside each cluster was less than the threshold of 1 watt, which is also referred to as the maximum allowable UAV transmit power, P U AV max . The performance comparison of different combinations of the clustering and efficient UAV 3D placement algorithms were evaluated, in terms of the minimum total UAV required transmit power, UAV placement and algorithm execution time.
For the uniform distribution scenario, the proposed power-efficient algorithm was evaluated against the multi-UAVs deployment approach that utilized CPT as described in Section 4.   Table 3 shows the simulation results of the proposed algorithm that minimizes the number of UAVs in providing wireless coverage to all users within the disaster-affected area, S, such that the UAV required transmit power is minimized and the users' data requirements are satisfied. The number of UAVs in Table 3, indicates the number of clusters.

Uniform Distribution Users Scenario
It can be observed that the power-efficient algorithm that invoked the PSO-based clustering algorithm outperforms the algorithm that invoked the K-means clustering algorithm. More specifically, six UAVs are required to be employed in providing wireless coverage to users within the disaster-affected area, S, as evidenced by the required transmit power of each UAV is less than 1 watt. However, in the case of the proposed algorithm that invoked K-means clustering algorithm, it is observed that the required transmit power of U AV 4 is more than 1 watt. As explained in Section 4, the number of clusters will be increased by 1 if the threshold value of UAV required transmit power is exceeded. Thus, this indicates that more UAVs are needed when the K-means clustering algorithm is invoked.  Figure 7 shows three of the clustering results and the UAVs placements from Table 3. Figure 7a depicts the simulation results of the proposed algorithm that iteratively invoked the K-means clustering algorithm and the PSO-based efficient UAV 3D placement algorithms. Meanwhile, the simulation results of the proposed algorithm that iteratively invoked the PSO-based clustering algorithm and two of the variants of efficient UAV 3D placement algorithms, namely PSO-based and ABC-based efficient UAV 3D placement algorithms, are depicted in Figure 7b,c, respectively. In this case, the outdoor and indoor users are distributed uniformly, which are denoted by the circle and cross symbols, respectively. These figures illustrate that the users are partitioned into six clusters, and one UAV is assigned to each cluster, where its 2D position is denoted by star. The coordinate of the 3D placement of each UAV in Figure 7a-c is presented in Table 3. It can be observed from Table 3 that the total UAV required transmit power is at a minimum when the UAV altitude is close to the minimum UAV altitude of 60 m. This is because there is a trade-off between the UAV altitude, the probability of LoS and the coverage performance in terms of coverage radius [8]. For lower UAV altitudes, the probability of LoS between transmitter and receiver decreases due to the shadowing impact. Thus, it results in a decrease in the total UAV required transmit power of each UAV. Figure 7a,b illustrate the comparison performance of clustering results between Kmeans and PSO clustering algorithms. From these figures, it can be clearly seen that the K-means algorithm produces clusters that are not symmetric, as discussed in Section 5. Meanwhile, Figure 7b,c illustrate the comparison performance of the proposed powerefficient algorithms that invoked PSO and ABC algorithms, respectively. It can be seen that both combinations produce similar performances in terms of clustering results and the UAV placements. Meanwhile, the convergence speed of the proposed algorithm that iteratively invoked PSO-based clustering algorithm and PSO-based efficient UAV 3D placement for the six UAVs is shown in Figure 8. It can be observed from this figure that the proposed algorithms converge to the total UAV transmit power smaller than the threshold value of 1 watt within a few iterations.  Figure 8. The convergence speeds of the power-efficient algorithm that iteratively invoked PSO-based clustering and PSO-based efficient algorithms for UAV1 to UAV6 when users were uniformly distributed. Figure 9 illustrates the optimal results of packing four, five and six equal circles inside the coverage area, S, using the CPT-based benchmarker algorithm. The problem of packing n c identical and non-overlapped circles inside a unit square for 2 ≤ n c ≤ 22 was studied in [9]. The problem is formulated with the objective to maximize the radius r d of the packed circles such that the coverage density, d n , is maximized. The coverage density, d n , for packing n c identical circles was determined using Equation (7). It was observed that the maximum coverage density of 78.5% was achieved when n c = 4, 9 and 16 [9]. It was also observed that the large number of UAVs (indicated by n c ) did not necessarily result in the maximum coverage density [9]. Packing four, five and six circles into the coverage area, S, that has a square shape resulted in coverage density of 78.5%, 67.4% and 66.4%, respectively. Thus, the maximum achievable coverage density of packing n c equal circles in the coverage region is 78.5%, when n c = 4.
Readers who are specifically interested in the most efficient algorithm in finding the minimum number of UAVs that maximizes wireless coverage using CPT to serve users in large coverage areas with three different shapes, namely square, rectangle and circular regions, are referred to [9] for further information. More specifically, discussions on the problem of packing n c identical circles using CPT inside a unit square and three different shapes of coverage area are presented in [9].
The performance of the proposed power-efficient algorithm that invoked a PSO-based clustering algorithm is evaluated against the benchmarker algorithm with a CPT-based approach. The condition of the total required UAV transmit power to be smaller than the threshold value of 1 watt is also considered in the CPT-based benchmarker algorithm. The results in Table 4 reveal that all users were partitioned into five clusters when the CPT-based benchmarker algorithm was invoked. Therefore, the multi-UAVs deployment with the CPT approach resulted in a smaller number of UAVs being deployed in providing wireless coverage to all users, but at the expense of the coverage density of 67.4% in comparison with the 100% coverage density using the proposed power-efficient algorithm.

Non-Uniform Distribution Users Scenario
For the case of users that are non-uniformly distributed, similar observations were obtained from Table 5. More specifically, the power-efficient algorithm that invoked a PSO-based clustering algorithm is superior in comparison to the algorithm that invoked a K-means clustering algorithm, as presented in Table 5. This is evident as the required transmit power for U AV 3 is more than the threshold value of 1 watt when the K-means clustering algorithm was used to partition the users. Thus, this indicates more than six UAVs are required to be deployed in providing wireless coverage for all users within the coverage area in comparison to the algorithm that invoked the PSO-based clustering algorithm which requires six UAVs. Moreover, Figure 10 shows the convergence speed of the proposed power-efficient algorithm that iteratively invoked the PSO-based clustering algorithm and PSO-based efficient UAV 3D placement for the case of non-uniformly distributed users. A similar observation was obtained from this figure where the simulation results converge to the total UAV transmit power smaller than the threshold value of 1 watt within a few iterations for each UAV that serves each cluster of users. Figure 11 shows three of the clustering results and the UAV placements from Table 5. A similar observation was obtained from comparison of Figure 11a,b, where it can be clearly seen that the K-means algorithm produces clusters that are not symmetric, as discussed in Section 5. It can be seen from Figure 11b,c that the combination of the PSO-based clustering algorithm and the two variants of efficient UAV 3D placement algorithms, namely ABCbased and GA-based algorithms, produced similar performances in terms of clustering results and the UAV placements.
The corresponding computational complexity performances for the proposed algorithm in relation to Tables 3 and 5 is shown in Table 6. In both scenarios, the execution time is used to represent the power-efficient algorithm computational complexity in finding the minimum number of UAVs to provide wireless coverage to outdoor and indoor users that are distributed uniformly and non-uniformly. It can be observed in Table 6 that the power-efficient algorithm took the shortest execution time to find the minimum number of UAVs by iteratively invoking the PSO-based clustering algorithm and the efficient UAV 3D placement algorithm based on PSO, rather than employing the other two variants of efficient UAV 3D placement algorithms.
Although the employment of the K-means clustering algorithm resulted in a shorter execution time when the PSO-based efficient UAV 3D placement algorithm was invoked, the resultant number of UAVs that were required to be deployed was inferior to that when the PSO-based clustering algorithm was invoked, as discussed in Section 7.1.  Figure 11. (a-c) Placement of UAV1 to UAV6 when users were non-uniformly distributed.
Therefore, it was found to be beneficial to employ PSO-based clustering and a PSObased efficient UAV 3D placement algorithm for the case of users that were distributed uniformly and non-uniformly.

Conclusions
In this work, a power-efficient algorithm was proposed for minimization of the number of UAVs to be deployed as aerial base stations to serve outdoor and indoor users simultaneously, which led to maximum users' coverage using the minimum number of UAVs, such that the required UAV transmit power was minimized.
In this study, three variants of clustering algorithms based on the K-means algorithm, PSO and GA were employed. It was observed that meta-heuristic clustering algorithms, namely GA-based and PSO-based, were superior in comparison to the K-means algorithm that resulted in a smaller number of user clusters.
It was observed that the proposed algorithm that iteratively invoked the PSO-based clustering algorithm and the PSO-based efficient UAV 3D placement algorithm has the lowest computational complexity compared to the proposed algorithm that invoked the efficient UAV 3D placement algorithm based on GA and ABC. The computational complexity performance was manifested in terms of the algorithm execution time. More specifically, for the uniformly distributed users scenario, the proposed algorithm that iteratively invoked the PSO-based clustering algorithm and the PSO-based efficient UAV 3D placement algorithm took about 0.0971 s to find the minimum number of UAVs which were positioned at their efficient UAV 3D placements.
The proposed algorithm was evaluated against a benchmarker algorithm that utilized CPT and efficient UAV 3D placement for a multi-UAV deployment scheme. It was observed that the benchmarker algorithm required a lower number of UAVs to be deployed as aerial base stations, but at the expense of the coverage density of 67.4% in comparison with 100% coverage density using the proposed power-efficient algorithm. The employment of the proposed algorithm resulted in 100% coverage density, which was manifested by its achievement to serve all users within the disaster-affected area, S.